The invention relates to a pivoting transmission unit, and more precisely a power transmission unit mounted so as to swivel about a pivot axis, to drive in rotation at least one driven component about an axis of rotation which has to be capable of swivelling about the pivot axis, substantially perpendicular to the axis of rotation of the driven component. The transmission unit comprises a casing mounted so as to pivot on and preferably between two stationary and rigid side parts of a support by two coaxial bearings about the pivot axis and spaced at a distance from each other along this pivot axis.
The pivoting power transmission unit according to the invention can be used, in a non-limiting manner, as a power transmission unit fitted to aircraft on which the propulsion system is swivellable, such as dirigible balloons or aircraft of the type known as convertible, or again as a power transmission unit fitted to wind turbines.
In general terms, the pivoting power transmission unit according to the invention can be used in all swivellable propulsion systems or systems generating energy by rotation which, to operate satisfactorily, must have no axial movement (in the direction of the pivot axis).
In particular, as an application for which the pivoting power transmission unit according to the invention is of great relevance for the applicant, such a transmission unit can be used to drive in rotation, from at least one power unit, of the turboshaft engine type, for example, at least one tilting rotor of an aircraft of the convertible type, which can operate in aeroplane mode or in helicopter mode, and in which the rotor drive shaft swivels about a pivot axis so that it can move from one position, in which the rotor operates as an aircraft propeller, for the aircraft to fly in aeroplane mode, to a position in which the rotor operates as a helicopter lift rotor, for the aircraft to fly in helicopter mode.
A pivoting power transmission unit for such a convertible aircraft with tilting rotors is described in particular in FR 2 791 319 and FR 2 791 634 to which reference should be made for further details.
It is pointed out however that these two patents describe a convertible aircraft with tilting rotors of the type in which the shaft of each rotor is driven in rotation about itself by a pivoting front reduction gear unit of one respectively of two power transmissions each also comprising a stationary rear reduction gear unit linked to the corresponding front reduction gear unit, and also to one respectively of two (turboshaft) engines, each supported by one respectively of the two fixed wings of the aircraft, and to an interconnecting shaft linking the two transmissions for the two rotors to be driven in rotation by any one of the two engines in case of failure of the other engine.
The shaft of each rotor, the corresponding power transmission and the corresponding engine are housed in one respectively of two articulated cowlings, each comprising a front part mounted, so as to pivot about the pivot axis, on a stationary rear part, fixed to one respectively of the two fixed wings of the aircraft, and in which are housed the corresponding engine and at least in part the rear reduction gear unit of the corresponding transmission, the front reduction gear unit of which, embodied as a helicopter main gearbox, and also the shaft of the corresponding rotor are housed in the pivoting front part of the cowling, and are mounted so as to pivot with this front part of the cowling relative to the stationary rear part of the cowling and the corresponding fixed wing.
In this application and with this architecture of each power transmission, the swivelling joint of the pivoting front reduction gear unit operates over only a limited angular sector of pivoting of about 110xc2x0, with a low speed of rotation of between about 1 and about 2 rpm, and under static load.
Consequently, the two bearings by which the casing of the swivelling front reduction gear unit or pivoting power transmission unit is mounted so as to pivot on the support, namely the stationary rear part of the cowling, are subject to false Brinelling, which is particularly noticeable when the bearings are rolling-element bearings, of the needle or roller bearing type.
The problem at the basis of the invention is to reduce if not completely eliminate this false Brinelling in the bearings and, in general terms, to propose a pivoting mounting of the power transmission unit on its support which is better suited to the various requirements of actual practice than the embodiments of the state of the art, and more precisely which offers better dynamic behaviour of the propulsion system in which such a pivoting power transmission unit is incorporated, in particular by substantially reducing the axial play, and therefore the movements of this propulsion system, along the pivot axis and also, preferably, the pressures in the bearings mentioned above.
To this end, there is provided in accordance with the present invention a pivoting power transmission unit for transmitting rotations drive to at least one driven component configured to revolve about an axis of rotation and configured to be swivelled about a pivot axis substantially perpendicular to said axis of rotation, said transmission unit comprising a casing mounted so as to pivot relative to two stationary and rigid side parts of a support by two coaxial bearings about said pivot axis and spaced at a distance from each other along the pivot axis, wherein the two bearings are plain bearings, each of which comprises:
a stationary part integral with one respectively of the two side parts of said support, and comprising a substantially cylindrical sleeve;
a swivelling part integral with said pivoting casing and comprising a substantially cylindrical annular trunnion mounted so as to swivel about said sleeve of the stationary part of said bearing; and
a wear take-up ring, based on a material with a low coefficient of friction, and comprising a cylindrical tubular part held axially between said sleeve of said stationary part and said trunnion of said moving part, and a radial collar projecting radially outwards from said tubular part and relative to its axis, and integral with one axial end of said tubular part which faces towards the respective side part of the support;
and wherein the transmission unit further comprises a device to take up play along the pivot axis, associated with at least one bearing so as to push the wear take-up ring and swivelling part of its corresponding bearing axially away from its respective side part of the support.
The substantially cylindrical sleeves of the stationary parts of the two bearings may be substantially coaxial with the pivot axis, and may extend towards each other from the side parts of the support. The substantially cylindrical annular trunnions of the swivelling parts of the two bearings may be substantially coaxial with the pivot axis.
In an advantageously simple, compact and economical mode of embodiment, the play take-up device comprises at least one annular piston, fitted so as to slide with sealing, along the pivot axis, in at least one annular chamber made in a stationary side part of said support and opening towards the wear take-up ring of said bearing associated with said play take-up device, said annular chamber being fed with fluid under pressure, for example hydraulic fluid, so that said annular piston bears axially against the radial collar of said wear take-up ring, which pushes the swivelling part of said bearing and said pivoting casing relative to the support, along the pivot axis.
A circuit for pressurising the annular chamber of the play take-up device may be provided but, advantageously, this annular chamber is fed with fluid under pressure, for example hydraulic fluid, by at least one tapping from at least one lubrication circuit of said transmission unit, for example.
Moreover, in order not to introduce a resisting torque when the transmission unit pivots, the annular piston advantageously has a bearing face at least partly covered with a coating based on a material with a low coefficient of friction, by which the piston bears against said collar of said wear take-up ring.
In this case, it is advantageous for the coating of the piston and the wear take-up ring to be based on the same material with a low coefficient of friction, for example PTFE.
Moreover, for better operation of the play take-up device together with a better pivoting mounting of the casing provided by the bearings, the wear take-up ring of at least one bearing, and preferably of each of them, is fixed to the swivelling part of said bearing.
The wear take-up ring is fixed to the swivelling part of the corresponding bearing, in an advantageously simple manner, by at least three screws passing, in a direction substantially parallel to the pivot axis, through said radial collar, in an area situated outside the area of contact with the annular piston, and screwed into said swivelling part of said corresponding bearing.
Also advantageously, in order to reduce the Hertz pressures in the bearings, the tubular part of the wear take-up ring of at least one bearing, and preferably of each of them, has an inner radial contact surface with a shape which is slightly cambered and convex towards the pivot axis, the location and amplitude of the convex cambered shape being such that said inner radial contact surface assumes a substantially cylindrical shape of circular cross-section under the load, especially the static load, applied to said pivoting casing.
Moreover, in order also to reduce the pressure peaks at the axial ends of the inner radial contact surface of the wear take-up ring of least one bearing, and preferably of each of them, this inner radial contact surface has a relieved area in the form of a chamfer or inclined flat at one at least of its two axial ends, and preferably at each of said two axial ends.
Finally, for application to a convertible aircraft with tilting rotors, the bearings are stressed over a limited angular sector of pivoting, of about 110xc2x0, with a low speed of rotation of between about 1 and about 2 rpm, of said pivoting casing under static load, about the pivot axis.